Since the early part of the 1960's, color television signals using the NTSC format have been broadcast throughout the world. This NTSC format has been the standard everywhere, and continues to be so today and on into the future. The NTSC format requires designated scan lines, bandwidths, etc., and utilizes a composite video signal having a luminance component (brightness) and a chrominance component (color).
The screen on a television is composed of hundreds of pixel elements which are activated according to the information received from the composite NTSC video signal. The color on these pixel elements is constantly changing over time as well as from pixel element to pixel element. One of the challenges to designers is how to compare the color changes occuring from pixel element to adjacent pixel elements to generate a clearer, sharper picture. The problem is further compounded as the element moves across the screen.
One important aspect of processing the composite video signal is separating the luminance component of the signal from the chrominance component. Without proper separation of the two components, problems in the display such as cross color dot crawl and color smear between pixel elements will occur.
Key to defining a sharper, clearer picture is defining a chrominance component free of luminance components, and in the alternative, a luminance component free from chrominance components. The closer the output of the separated chrominance (or luminance) component comes to the actual chrominance (or luminance) spectrum, the better the picture.
It has been discovered that passing the composite video signal through a band-pass filter (BPF) will remove most of the low frequency luminance components yielding a chrominance component having some high frequency luminance signals remaining. Further, passing the composite video signal through a comb filter will entirely eliminate most of the high frequency luminance components as well as leaving a chrominance output generally matching the actual shape of the chrominance component of the video signal. To obtain the luminance component, the chrominance is subtracted from the composite video signal.
At first glance it would appear that the ideal situation would be to always pass the composite video signal through a comb filter to achieve the chrominance component. However, it has further been realized that when there is a significant change in chrominance between pixel elements in a vertical direction, that is the chrominance changes significantly from pixel element to pixel element in a vertical line, a combed chrominance signal will result in a phenomenon called dot-crawl and will decrease the quality of the picture. In this case, passing a composite video signal through a BPF resulting in a chrominance component with high frequency luminance signals will result in a better quality picture than passing the signal through a comb filter.
Accordingly, circuits have been designed where the difference in the chrominance component in a vertical direction determines whether to use a comb filter or a BPF to obtain the chrominance component. Two types of chrominance/luminance separation circuits have been developed. The first, used in high end products and costing substantially more than the other type, is called a 2 H delay circuit since it uses three different scan lines in the separation process: no delay, one delay, and two delays. This 2 H delay circuit has an output quite close to the shape of the chrominance/luminance signal.
The second type of chrominance/luminance separation circuit is a 1 H delay circuit (for purposes of the present invention, a 1 H delay circuit is a digital delay circuit in contrast to an analog delay circuit which serves the same function) since only two scan lines are compared: a scan line having only 1 delay and the no-delay scan line. The output does not approximate the actual chrominance/luminance signal as close as the 2 H delay circuit. However, the quality of the picture using a 1 H delay circuit is considered sufficient for the cost.
For 1 H separation circuits, it is desirable to develop a comb filter that outputs a chrominance signal that more closely resembles that of the actual chrominance component.